1. The Field of Invention
The present invention relates to a method and apparatus for uniformly and continuously drawing samples of gas entrained in a liquid containing a high percentage of solids. More particularly, the present invention relates to a method and apparatus for obtaining samples of gases contained in drilling mud coming to the surface from an oil well drilling operation.
2. The Prior Art
The conventional practice in drilling for oil is to use a special fluid, termed "drilling mud", which is pumped down the drill string to circulate from the drilling head and carry upward to the surface the debris created by the drilling operation. When a gas-containing strata is encountered by the drilling operation, a certain amount of the gas from the strata will be entrained in the drilling mud and thus be carried to the surface. Extracting these gases from the drilling mud allows determination of the presence of hydrocarbons and an estimate of the quantity of hydrocarbon being encountered. Analysis of the recovered gas can be used to make a determination as to the desirability of recovering the gas or oil from the particular strata. This practice is generally categorized as "mud logging". The known devices for accomplishing mud logging separate the gas from the fluid by an agitation or vibrating process. The gas, i.e. hydrocarbon, samples are collected in a gas trap during this operation. Gas traps of several different designs are currently used in the mud logging industry in order to extract light hydrocarbon gases from the return flow line mud for measurement. The purpose of this measurement is twofold: (1) to provide warning of dangerous underbalanced drilling conditions indicated by increased gas returns; and (2) to evaluate the formation being drilled for hydrocarbon productivity.
Several different gas traps are currently used in the mud logging industry. The purpose of these trap systems is to measure the amount of gas in the drilling fluid, which gas will be representative of the formation gas. This measurement is critical to identification of productive zones during drilling of the well. However, existing traps are not reliable and are very dependent upon operating conditions, such as mud flow rate and air dilution of the sample as it passes through the trap. These parameters cannot be readily controlled by many existing trap designs. The fluid level, where the trap is installed, will change during the drilling operation. This change in level will affect the flow of fluid through the trap thereby changing the amount of gas measured by the trap over any given time period while there is no actual change in the amount of gas in the drilling fluid.
The amount of air dilution cannot be measured accurately in the current traps because of air and gas leaks through the fluid exhaust port, which is generally open to the air outside the trap, and leakage around the motor shaft stirrer bar.
In general, gas traps operate by diverting a portion of the return mud through an enclosed volume which provides some mechanism for gas release within that volume. The mechanism may be passive, such as a mud-spreading plate, or may contain some sort of mechanical agitator to maximize the mud/air contact. In either case, the evolved gas is conveyed to the analytical equipment by means of suction applied to a gas phase sample line attached to the trap body. Due to the need to provide continuously updated gas readings, mud residence time within the trap is normally so short that only a fraction of the gas is released. For quantitative operation, the trap design must therefore be such that the observed gas in the sample stream can be easily related to the actual gas content of the return mud.
Regardless of the details of the trap operation, several flows are always present in one form or another, mud phase entry and exit flows to permit continuous sampling of fresh mud, gas phase sampling flow, and gas phase vent flow whose direction and rate is determined by the difference in gas evolution and gas sampling rates. In order for quantitative reproducible readings to be obtained, these flows should be discreet and accessible to measurement by the operator. Of particular importance is the avoidance of uncontrolled external air and evolved gas mixing due to poor design of the trap vent flow, a failing encountered in several commonly used trap designs.
Another fault in many designs is the excessive variation in trap response with changes in the return mud level. The driller often has occasion to change the pump rate. Such flow rate changes alter the level of mud in the return mud handling equipment and, unless the trap is dynamically mounted, also alter the immersion level of the trap mud entry port. One solution sometimes is to provide an active pumping mechanism in the trap, but, due to formation cuttings in the mud, such pumps are prone to jamming and high maintenance requirements with the attendant high costs.